1. Field of the Invention
The invention relates to a method, a computer program product, a data medium and a computer system for grapheme-phoneme conversion of a word which is not contained as a whole in a pronunciation lexicon.
2. Description of the Related Art
Speech processing methods in general are known, for example, from U.S. Pat. No. 6,029,135, U.S. Pat. No. 5,732,388, DE 19636739 C1 and DE 19719381 C1. In a speech synthesis system, the script-to-speech conversion or grapheme-phoneme conversion of the words to be spoken is of decisive importance. Errors in sounds, syllable boundaries and word stress are directly audible, can lead to incomprehensibility and can, in the worst case, even distort the sense of a statement.
The best quality speech recognition is obtained when the word to be spoken is contained in a pronunciation lexicon. However, the use of such lexica causes problems. On the one hand, the number of entries increases the search outlay. On the other hand, it is precisely in the case of languages such as German that it is impossible to cover all words in a lexicon, since the possibilities of forming compound words are virtually unlimited.
A morphological decomposition can provide a remedy in this case. A word which is not found in the lexicon is decomposed into its morphological constituents such as prefixes, stems and suffixes and these constituents are searched for in the lexicon. However, a morphological decomposition is problematical precisely in the case of long words, because the number of possible decompositions rises with the word length. However, it requires an excellent knowledge of the word formation grammar of a language. Consequently, words which are not found in a pronunciation lexicon are transcribed with out-of-vocabulary methods (OOV methods), for example, with the aid of neural networks. Such OOV treatments are, however, relatively compute-intensive and generally lead to poorer results than the phonetic conversion of whole words with the aid of a pronunciation lexicon. In order to determine the pronunciation of a word which is not contained in a pronunciation lexicon, the word can also be decomposed into subwords. The subwords can be transcribed with the aid of a pronunciation lexicon or an OOV method. The partial transcriptions found can be appended to one another. However, this leads to errors at the break points between the partial transcriptions.